Recently, biological treatment techniques of waste water have been widely used for preventing water pollution resulting from the accumulation of organic substances. Known biological treatment methods of waste water typically include: (1) an activated sludge type process; and (2) a biological fixed film type process.
Among these processes an activated sludge type process in which an aqueous suspension of microorganisms is utilized is most prevalent for purifying waste water such as sewage, various kinds of industrial waste water and the like.
An apparatus for a typical activated sludge type process is shown in FIG. 1. Waste water containing organic substances is fed via an influent(input) line (32) by a pump (31) into aeration tanks (3a, 3b, 3c) and the organic substances is degraded by the microorganisms grown in the tanks(3a, 3b, 3c). Air is supplied through a blower (4) into the aeration tanks (3a, 3b, 3c) and dissolved into the waste water. The treated water flows into a final clarifier (6a, 6b) through an effluent line (6), is separated from the activated sludge, and is discharged through an overflow pipe (7). The precipitated activated sludge is returned to the aeration tank (3a) through a pump (18) and a part of the sludge is wasted through a pipe (19) to maintain a constant MLVSS(Mixed-liquor Volatile Suspended Solids) value in the aeration tanks (3a, 3b, 3c).
A conventional activated sludge process for treating a organic waste water containing a high concentration of organic substances was of a long-term aeration type and it is difficult to maintain constant operation conditions or a high efficiency of operation during the long period of operation.
Particularly, in the case of a treatment system for treating a waste water produced from petrochemical industries which usually contains a large amount of organic substances and heavy metals, the overloading causes a toxic effect to microorganisms in the aeration tank so that the treatment efficiency is reduced, or the process becomes shut down. Under such a circumstances, the loading rate should be reduced, which consequently results in an insufficient treatment of waste water and water pollution. Moreover, since the influent(input) flow rate maintains constant regardless of variations in the concentration of organic substances in the waste water, the variations in the concentration of organic substances result in a fluctuation of the loading rate, which give the microorganisms a "shock". These effects of the organic concentration on the microorganisms can be detected only after they appear. Moreover, it takes at the least 30 minutes and at the most 5 days to measure the organic concentration parameters such as COD(Chemical Oxygen Demand), BOD.sub.5 (Biological Oxygen Demand in 5 days), and TOD(Total Oxygen Demand), which indicate the environmental conditions in the aeration tanks. Accordingly, the effect of the variation of organic concentrations can be removed only several days after they are detected.
Therefore, there has been a need to develop a method and apparatus which enable to operate the waste water treatment plant efficiently and produce an environmentally acceptable effluent by controlling the influent flow rate based on the relationship between an input loading rate (organic concentration.times.input flow rate) and an actual respiration rate.
There have been several attempts to solve the problems. For example, JP 55-97290A discloses an automatic influent flow rate control system wherein the waste water influent flow rate into the activated-sludge aeration tank is reduced when the measured DO(Dissolved Oxygen) in the aeration tank is lower than a certain value and is increased when the DO becomes above the certain value. JP 2-68196A discloses a loading control system for a waste water treatment apparatus having a plurality of unaeration tanks in which the waste water is fed and a plurality of aeration tanks which is connected to the unaeration tanks, wherein a set DO value is given for each aerobic tank and a flow rate of influent into the unaeration tanks is determined based based on the capacity of the tanks. The influent flow is controlled depending on the set DO. This is possible by accumulating the influent flow rates measured by flow meter and propionic acid concentrations in the effluent line of the anaerobic reactor.
However, the JP 55-97290A has some drawbacks that since the system controls the influent flow rate depending on the DO in the aeration tank, there is a long lag time between the measurement of the DO and control of the flow rate, and this fails to timely response to the over-loading or shock.